It is well known that natural and synthetic rubbers usually must be mixed or "compounded" with vulcanizing agents, plasticizers, extenders, fillers, pigments, and the like, so that the rubber can be cured or vulcanized in a mold to form useful articles. It has frequently been found necessary to incorporate certain "processing aids" in the rubber compound prior to molding and curing to improve the mixing of the ingredients of the rubber compound, the flowability of the rubber during processing, and the extruding characteristics of the rubber product, without seriously adversely affecting the physical properties of the rubber compound. In addition, such aids are employed to reduce the mixing time requisite to effect rubber compounding, to enable the use of lower mixing temperatures and to provide for better additive dispersal than do traditional methods.
In accordance with U.S. Pat. No. 3,787,341, issued Jan. 22, 1974, it was found that useful processing aids include homogenous mixtures of
1. alkali metal or amine salts of aromatic sulfonic acids of the general formula: ##STR1## wherein X can be either hydrogen or a branched or straight chain alkyl group having from 4 to 14 carbon atoms, and preferably 8 to 12 carbon atoms; and n has a value of 1 to 5, and preferably 1; and
2. long chain fatty acids having a maximum of about 22 carbon atoms in the chain, such as, but not limited to, stearic acid, palmitic acid, oleic acid, neodecanoic acid and mixtures of such acids, present in amounts at least about equal to the amount of sulfonate present.
As is disclosed in U.S. Pat. No. 3,882,062, issued May 6, 1975, it was subsequently found that thioethers catalyze the peptizing action of the above composition in achieving a reduction in the viscosity of the rubber compound. This catalysis provides for a reduction in the amount of energy required to effect compounding and allows the mixing of the rubber compound to occur at lower temperatures, thereby reducing the possibility of scorching.
It was then found in accordance with U.S. Pat. No. 4,010,129, issued Mar. 1, 1977, and copending application Ser. No. 920,952 filed June 30, 1978, that thioethers not only catalyze the action of the above specified fatty acid/aromatic sulfonate composition, but also provide for the reduction and even the elimination of the fatty acid ingredient without detrimentally affecting either the physical properties or the processing of the rubber compound. Through the realization of this invention, a substantial savings in the cost of the processing aid is achieved.
A preferred class of thioethers disclosed in U.S. Pat. No. 4,010,129, issued Mar. 1, 1977, consists of the thiodicarboxylic acids and their corresponding lower alkyl ethers of the formula: ##STR2## wherein Y is as defined above; z is a number whose value falls in the range of 1 to about 2; y is a number whose value ranges from 0 to about 1; and R.sup.3 is hydrogen or a lower alkyl containing a maximum of 3 carbon atoms. Suitable thioethers of this class include thiodiglycolic acid, thiodipropionic acid, methylene bis(thioacetic acid), dimethyl thiodipropionate, thiodisuccinic acid, thiodipropionitrile and dibenzyl sulfate.
Copending application Ser. No. 920,952 entitled "Novel Processing Aids for Natural and Synthetic Rubber Compounds" filed June 30, 1978, discloses that especially desirable thioethers for use in this regard are thioethanols of the general formula: ##STR3## wherein a is a number whose value is either 0 or 1 and b is a number whose value is either 1 or 2, such that the sum of a and b equals 2; c is a number whose value is either 1 or 2; A represents a hydrocarbon radical with 1 to 18 carbon atoms; and B, D, E and F represent either hydrogen or hydrocarbon radicals with 1 to 18 carbon atoms, all of the above with the proviso that the total number of carbon atoms in the thioether does not exceed 21.
Hydrocarbon radicals preferably comprise saturated or aromatically unsaturated moieties, and include alkyl, aryl, alkaryl and aralkyl radicals. The radicals represented by B, D, E and F are preferably hydrogen or lower alkyl of from about 1 to about 3 carbon atoms. Typical thioethanols which were disclosed include 2-(methylthio)ethanol, 2-(ethylthio)ethanol, 2-(2'-ethylhexylthio)ethanol, 2-(phenylthio)ethanol, 2-(dodecylphenylthio)ethanol, 2-(benzylthio)ethanol, 2-(ethylthio)-1-methylethanol and the like. Preferred thioethanols are those wherein b is 2, with thiodiethanol being especially preferred.
Certain problems arise in the formation of the amine salts of aralkylsulfonic acids to be used as rubber processing aids in accordance with the foregoing patents and patent application. The formation of the aralkylsulfonate salt requires mixing of the sulfonic acid with an amine, and the corrosive nature of the acid often results in substantial damage to the mixing equipment during this process. In connection with the use of the aralkylsulfonate salt composition, ease of handling and measurement of precise amounts is hindered by the usual physical states of the compositions, e.g., as waxy or hard solids.
It has been found in accordance with my copending application Ser. No. 888,605, filed Mar. 20, 1978, that rubber processing aids composed of petrolatum and natural or synthetic clays in the form of particulate, relatively free-flowing powders are easier to handle and more efficient to use than traditional forms of such processing aids. In addition, the use of a clay substrate provides a medium for combining any immiscible liquids used in the processing aid.